The invention relates to a power supply system with two batteries of different voltages and at least one load device which is connected in parallel with both batteries, such that if one battery fails, the other battery assures the supply of power. Such duplicate power supplies are gaining interest in areas such as automobiles, where it is necessary to assure operation of apparatus critical to safety.
FIG. 4 shows a block circuit diagram of a conventional power supply system. Voltage terminals 2 and 4 of an electronic control apparatus 6 are connected each through fuses 8 and 10, respectively, to batteries 12 and 14, respectively. Battery 12 in the example shown has a voltage of 12 V and battery 14 has a voltage of 42 V. Each voltage terminal leads to power supply centers 16 and 18, respectively, which are matched to a particular battery voltage. Each power supply center is connected to an electronic control unit 20 whose output signal operates, through an output 22, a switch 24 which is contained in a unit 26 to be controlled by the control apparatus 6.
The power is supplied to the unit 26 via power supply lines 28 and 30, each connected to one of the two batteries 12 and 14, respectively, and connected by diodes 32 and 34, respectively, to the input terminal of switch 24.
A ground conductor 36 runs from the battery 14 to the ground connection of the control apparatus 6 and to the ground connection of the unit 26. The ground of battery 12 is connected by an additional ground line, which can be the vehicle body itself, to the ground of the control apparatus 6.
The operation of this conventional power supply system is well known in the art. If either of the two batteries 12 or 14 fails, the remaining battery supplies the power. The diodes 32 and 34 and/or the power supply centers provide assurance that current flows only from the operational battery to the unit 26 and prevents the occurrence of a short circuit discharge.
In order to reduce cost and weight, and to improve safety, it is desirable to eliminate the additional ground connection 36, which is typically designed with a large cross section.
Additionally, the diagnosis of the two separate grounds can be difficult and is prone to errors.
A short circuit in the lines of connected units or sensors (not shown) should have as little effect as possible on the operation of the overall system.
Another conventional power supply system is disclosed in DE 196 02 171 A1. According to this power supply system, the power input of a voltage regulator is connected through a double diode to both a main line connected to the positive pole of a battery and to a secondary line which is connected through an ignition switch to the positive pole of the on-board network. A buffer condenser is connected to the secondary line and all grounds of the system are brought together. In the event of a voltage collapse on the main line, the power is supplied through the secondary line with current flowing from the buffer condenser. If the voltage on the secondary line collapses, power is supplied through the buffer condenser, which is of sufficient size to buffer the power input.
Yet another conventional power supply system is disclosed in DE 196 45 944 A1. This power supply system contains a controller for an on-board network with at least two batteries which serve to power various load devices. The controller is between the two batteries and contains a power supply center which is connected through diodes to the two batteries. The system has a common ground. The controller also has a bidirectional connection for a bus, such as a CAN bus.
In the event of interruptions of the ground connection between the batteries, failures, which are difficult to diagnose, can occur in the load device of the conventional power supply systems described above.